Flexible display device

ABSTRACT

A flexible display device of the present invention comprises a photoelectric element part; a first adhesive film formed on the photoelectric element part; a touch function part formed on the first adhesive film; a second adhesive film formed on the touch function part; and a window film formed on the second adhesive film; wherein the first adhesive film or the second adhesive film has an average slope of about −9.9 to about 0 at −20° C. to 80° C. based on x-axis for the distribution of storage modulus versus temperature in the function of x-axis of temperature (° C.) and y-axis of storage modulus (KPa) and wherein the first adhesive film or the second adhesive film has a storage modulus of about 10 KPa or more at 80° C.

This application is a National Phase patent application and claimspriority to and the benefit of International Application NumberPCT/KR2015/011603, filed on Oct. 30, 2015, which claims priority to andthe benefit of Korean Application No. 10-2014-0153211, filed on Nov. 1,2014, Korean Application No. 10-2014-0150799, filed on Nov. 1, 2014,Korean Application No. 10-2014-0156463 filed on Nov. 11, 2014, andKorean Application No. 10-2015-0150670 filed on Oct. 29, 2015, theentire contents of each of which are incorporated herein by reference.

BACKGROUND 1. Field

The present invention relates to a flexible display device.

2. Description of the Related Art

Recently, glass substrates or high hardness substrates have beenreplaced with films in optical display devices such as liquid crystaldisplay devices, organic light emitting diode display devices, and thelike. Thus, flexible display devices having flexibility, which can befolded and unfolded, have been developed. As the films are used as asubstrate in the flexible display devices, it has an advantage that itis thin and light, and has high impact resistance, and can be folded andunfolded thus fabricated in a various form.

The flexible display devices have a structure in which a window filmsare laminated on display elements. An optical clear adhesive (OCA) filmis intervened between the display elements and window films.

The usage environments, storage environments and/or preparationenvironments, and the like of the optical display devices have recentlybeen harsh. In addition, a variety of physical properties that has beenrequired as a concern on the flexible optical display devices have beengradually increased. Particularly, to apply them to the flexibledisplays, the OCA films capable of maintaining viscoelasticity propertyin wide temperature range and having excellent recovery property arerequired.

Japanese Laid-Open Patent No. 2007-176542 discloses the background ofthe present invention.

SUMMARY

The object of the present invention is to provide a flexible displaydevice having excellent quality by applying an adhesive film havingexcellent viscoelasticity property and recovery property.

The other object of the present invention is to provide a flexibledisplay device capable of maintaining excellent reliability anddurability even though in the harsh usage environments, storageenvironments and/or preparation environments.

These and other objects of the present invention will be achieved byusing aspects of the present invention described below.

In one aspect of the present invention, provided is a flexible displaydevice. In one embodiment the flexible display device including aphotoelectric element part; a first adhesive film formed on thephotoelectric element part; a touch function part formed on the firstadhesive film; a second adhesive film formed on the touch function part;and a window film formed on the second adhesive film; wherein the firstadhesive film or the second adhesive film has an average slope of about−9.9 to about 0 at −20° C. to 80° C. of x-axis for the distribution ofstorage modulus versus temperature in the function of x-axis oftemperature (° C.) and y-axis of storage modulus (KPa) and the firstadhesive film or the second adhesive film may have a modulus of about 10KPa or more at 80° C.

In another embodiment the flexible display device including aphotoelectric element part; a first adhesive film formed on thephotoelectric element part; a touch function part formed on the firstadhesive film; a second adhesive film formed on the touch function part;and a window film formed on the second adhesive film; wherein the firstadhesive film or the second adhesive film is formed from anpressure-sensitive adhesive composition including a monomer mixture fora (meth)acrylic copolymer having a hydroxyl group; and organicparticles, and the organic particles have an average particle diameterof 10 nm to 400 nm.

The flexible display device may further include a polarizer formed onthe touch function part or on the first adhesive film.

The touch function part may include a substrate layer, and the substratelayer may include a polarizer.

The photoelectric element part may be an OLED, a LED or a LCD includinga light source.

The first adhesive film may have a thickness greater than that of thesecond adhesive film.

The first adhesive film or the second adhesive film may have a T-peelstrength of about 400 gf/in to about 5,000 gf/in, as measured at 25° C.with respect to a corona-treated polyethylene terephthalate (PET) film.

The first adhesive film or the second adhesive film may have a T-peelstrength of about 200 gf/in to about 3,000 gf/in, as measured at 60° C.with respect to a corona-treated polyethylene terephthalate (PET) film.

The first adhesive film or the second adhesive film may have a storagemodulus of about 10 KPa to about 1000 KPa at 80° C.

The first adhesive film or the second adhesive film may have a storagemodulus of about 10 KPa to about 1000 KPa at −20° C.

The first adhesive film or the second adhesive film may have a haze ofabout 5% or less after 200% stretching at a thickness of 100 μm.

The first adhesive film or the second adhesive film may have a recoveryrate of about 30% to about 98% at a thickness of 100 μm according tofollowing Equation 1.

Recovery rate (%)=(1−(X _(f) /X ₀))×100  [Equation 1]

(where in equation 1, when both ends of each of polyethyleneterephthalate (PET) films (thickness: about 75 μm) having a size ofabout 50 mm×about 20 mm (length×width) are defined as a first end and asecond end, respectively, a specimen is prepared by bonding ends of twoPET films to each other via an adhesive film having a size of about 20mm×about 20 mm (length×width) in order of first end of first PETfilm/adhesive film (length×width: about 20 mm×about 20 mm)/second end ofsecond PET film. Next, jigs are secured to non-bonded ends of the PETfilms of the specimen, respectively. Next, the jig at one side is keptfixed and the jig at the other side is pulled to a length (about 10times of thickness, X₀) of about 1,000% of thickness (unit: μm) of theadhesive film of the adhesive film at a rate of about 300 mm/min andthen maintained for about 10 seconds. When a force of about 0 kPa isapplied to the adhesive film by recovering the adhesive film at the samerate (about 300 mm/min) as the pulling rate, an increased length of theadhesive film is defined as X_(f) (unit: μm)).

The first adhesive film or the second adhesive film may have anelongation of about 800% to about 2000%.

The first adhesive film or the second adhesive film may have a bubblegeneration area of about 0%, as measured when the adhesive film (about13 cm×about 3 cm, about thickness of about 100 μm) comprising a about 50μm thick PET film stacked on one surface thereof and a about 100 μmthick PET film stacked on the other surface thereof is bent towards theabout 50 μm thick PET film such that the adhesive film has ½ of thelength, followed by placing the adhesive film between parallel frameshaving a gap of about 1 cm, and then subjected to aging under conditionsof about 70° C. and humidity of about 93% for about 24 hours.

The first adhesive film or the second adhesive film in the flexibledisplay device may have a bubble generation area of 0%, as measured whenthe flexible display device that a window film was replaced with PETfilm (thickness of about 100 μm), bended into the parallel frames havinga spacing of about 1 cm in the direction of the photoelectric elementpart, and aging at temperature of about 70° C. and humidity of about 93%for about 24 hours.

The first adhesive film or the second adhesive film may be a curedproduct of the pressure-sensitive adhesive composition, and thepressure-sensitive adhesive composition may include a monomer mixturefor a (meth)acrylic copolymer having a hydroxyl group, and organicparticles.

The organic particles may have an average particle diameter of about 10nm to about 400 nm.

The monomer mixture including a (meth)acrylate monomer having thehydroxyl group and a comonomer.

The organic particles may have core-shell structures, and a glasstransition temperature of the core and the shell may satisfy thefollowing Equation 2.

Tg(c)<Tg(s)  [Equation 2]

(Where in equation 2, Tg(c) is a glass transition temperature (° C.) ofthe core, and Tg(s) is a glass transition temperature (° C.) of theshell).

The core may have a glass transition temperature of about −150° C. toabout 10° C., and the shell may have a glass transition temperature ofabout 15° C. to about 150° C.

The core may include at least one polyalkyl (meth)acrylate having aglass transition temperature of about −150° C. to about 10° C., and theshell may include at least one polyalkyl (meth)acrylate having a glasstransition temperature of about 15° C. to about 150° C.

The core or the shell may include two or more of layers, and theoutermost layer of the organic particles may include at least onepolyalkyl (meth)acrylate having a glass transition temperature of about15° C. to about 150° C.

The shell may be present in an amount of about 1 wt % to about 70 wt %in the organic particles.

The organic particles may be present in an amount of about 0.1 parts byweight to about 15 parts by weight based on 100 parts by weight of themonomer mixture for the (meth)acrylic copolymer having the hydroxylgroup.

The organic particles may have a refractive index difference of about0.05 or less from that of the (meth)acrylic copolymer having thehydroxyl group.

The (meth)acrylic copolymer having the hydroxyl group may be the polymerof about 60 wt % to about 95 wt % of the comonomer and about 5 wt % toabout 40 wt % of the (meth)acrylate having the hydroxyl group.

The comonomer may include at least one of an alkyl (meth)acrylatemonomer, a monomer having ethylene oxide, a monomer having propyleneoxide, a monomer having amine group, a monomer having amide group, amonomer having alkoxy group, a monomer having phosphate group, a monomerhaving sulfonic acid group, a monomer having phenyl group and a monomerhaving silane group, and the comonomer may have a glass transitiontemperature (Tg) of about −150° C. to about 0° C.

The pressure-sensitive adhesive composition may further include at leastone of an initiator and a crosslinking agent.

In accordance with aspects of the present invention, provided is aflexible display divice capable of maintaining excellent reliability anddurability and having excellent quality even though in the harsh usageenvironments, storage environments and/or preparation environments byapplying an adhesive film having excellent viscoelasticity property andrecovery property.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a flexible display device according to oneembodiment of the present invention.

FIG. 2 is a sectional view of a flexible display device according toanother embodiment of the present invention.

FIG. 3 is a sectional view of a flexible display device according tostill another embodiment of the present invention.

FIG. 4 is a conceptual diagram of a specimen for measuring T-peelstrength.

FIG. 5 shows sectional and plan views of a specimen for measuringrecovery rate.

DETAILED DESCRIPTION

Embodiments of the present invention will be described in detail withreference to the accompanying drawings. It should be understood that thepresent invention may be embodied in different ways and is not limitedto the following embodiments. In the drawings, portions irrelevant tothe description will be omitted for clarity. Like components will bedenoted by like reference numerals throughout the specification.

As used herein, terms such as “upper” and “lower” are defined withreference to the accompanying drawings. Thus, it will be understood thatthe term “upper” can be used interchangeably with the term “lower.” Theterm “on” encompasses both that one element is disposed “directly on”another structure, and that the other structure can be interposed.Meanwhile, the term “directly on” means that the other structure is notinterposed.

As used herein, the term “(meth)acryl” refers to acryl and/or methacryl.

As used herein, the term “copolymer” includes oligomers, poymers, orresins.

As used herein, the term “comonomer” is not limited, but may be amonomer which can be polymerized with a (meth)acrylate having a hydroxylgroup.

As used herein, the “glass transition temperature” may be determined,for example, on a homopolymer of each monomer to be measured using a DSCDiscovery from TA Instruments. Specifically, the glass transitiontemperature may be determined on the homopolymer of each monomer byincreasing the temperature to about 180° C. at a rate of 20° C./min andallowing it cool slowly to −80° C., thereby the data of an endothermictransition curve at time which the temperature increases to 180° C. at arate of 10° C./min, and the point of inflection of the endothermictransition curve may be taken as the glass transition temperature.

The term “foaming area of adhesive film” used herein means a valuemeasured on an adhesive film specimen prepared by transferring theadhesive film (about 13 cm×about 3 cm, thickness of about 100 μm) to aPET film having a thickness of about 50 μm, and adhering the PETsubstrate having a thickness of about 100 μm to the back side of theadhesive film. The adhesive film specimen is bended and putted into theparallel frames having a spacing of about 1 cm in the direction of PEThaving a thickness of 50 μm such that the width length of the specimenof the adhesive film becomes about ½, and aged at temperature of about70° C. and humidity of about 93% for about 24 hours. Then, the images ofthe area, in which the bubbles occur, taken with an optical microscope(Olympus, EX-51) are analyzed using a Mac-view software from MountechCo., Ltd., to calculate a ratio (%) of the area and size in which thebubbles occupy to the area of the specimen.

The term “foaming area of display device” used herein means a valuemeasured by specimen prepared by transferring the window film of theflexible display device to a PET film (thickness of about 100 μm),thereby specimen is bended and putted into the parallel frames having aspacing of about 1 cm in the direction of the PET film, and aging attemperature of about 70° C. and humidity of about 93% for about 24hours, and analyzing the images of the display device taken with anoptical microscope (Olympus, EX-51) using a Mac-view software fromMountech Co., Ltd., to calculate a ratio (%) of the area and size inwhich the bubbles occupy to the area of the display device.

The term “average particle diameter of organic particles” used hereinmeans a Z-average value measured in an aqueous or organic solvent usinga Zetasizer nano-ZS equipment from Malvern Ltd.

The term “core-shell structure” used herein refers to a typicalcore-shell structure. In addition, the core or the shell refers tomonolayer or multi-layer, respectively, and the “outermost layer” refersto the most outer layer in a number of layers.

The term “elongation” may be determined by rolling closely the adhesivefilm of a thickness 100 μm and a size of about 5 cm×about 5 cm, andfixing it to the both ends of a TA (TA.XT Plus Texture Analyzer (StableMicro Systems, Ltd)), then stretching it at a rate of about 300 mm/min,and elongation means a ratio of the length before the stretching to thelength at time which the adhesive film breaks by stretching(elongation).

As used herein, the term “T-peel strength” refers to a value measured bythe following procedures of i) to v):

i) An adhesive composition is coated onto a polyethylene terephthalate(PET) release film, followed by UV irradiation at a dose of about 2000mJ/cm², thereby manufacturing an about 100 μm thick adhesive sheet of anadhesive film and the PET film.

ii) A PET film, which has a size of about 150 mm×about 25 mm×about 75 μm(length×width×thickness) and is subjected to corona treatment twice(total dose: about 156) under corona discharge at a dose of about 78using a corona treatment device, is prepared.

iii) An adhesive film sample having a size of about 100 mm×about 25mm×about 100 μm (length×width×thickness) is obtained from the adhesivesheet, followed by laminating the corona-treated surfaces of the PETfilms to both surfaces of the adhesive film sample, thereby preparing aspecimen, as shown in FIG. 4(a).

iv) The specimen is subjected to autoclaving under conditions of about3.5 bar and about 50° C. for about 1,000 seconds and secured to aTA.XT_Plus texture analyzer (Stable Micro System Co., Ltd.).

v) In the TA.XT_Plus texture analyzer, the PET film at one side is keptfixed and the PET film at the other side is pulled at a rate of about 50mm/min, thereby measuring T-peel strength (see FIG. 4(b)).

Herein, the “recovery rate” can be measured through the followingprocedures: When both ends of each polyethylene terephthalate (PET) film(thickness: about 75 μm) having a size of about 50 mm×about 20 mm(length×width) are defined as a first end and a second end,respectively, a specimen is prepared by bonding ends of two PET films toeach other via an adhesive film having a size of about 20 mm×about 20 mm(length×width) in order of first end of first PET film/adhesivefilm/second end of second PET film, and has a contact area of about 20mm×about 20 mm (length×width) between each of the PET films and theadhesive film (see FIGS. 5(a) and 5 (b)). Referring to FIG. 5(a), jigsare secured to non-bonded ends of the PET films of the specimen at roomtemperature (about 25° C.), respectively. Next, the jig at one side iskept fixed, and the jig at the other side is pulled as much as a lengthof about 1,000% of thickness (unit: μm) of the adhesive film (10 timesan initial thickness (X₀) of the adhesive film) at a rate of about 300mm/min and then maintained for about 10 seconds. Next, if an increasedlength of the adhesive film is defined as X_(f) (unit: μm) when a forceof about 0 kPa is applied to the adhesive film by recovering theadhesive film at the same rate (300 mm/min) as the pulling rate, therecovery rate (%) is calculated by Equation 1:

Recovery rate (%)=(1−(X _(f) /X ₀))×100  [Equation 1]

Here, the initial thickness of the adhesive film may range from about 20μm to about 300 μm.

Recovery rate may be measured using a TA.XT_Plus texture analyzer(Stable Micro System Co., Ltd.). Recovery rate may be measured at about25° C. to about 80° C.

Hereinafter, a flexible display device of the present invention will bedescribed with reference to FIG. 1. FIG. 1 is a flexible display deviceaccording to one aspect of the present invention.

Referring to FIG. 1, a flexible display device 100 of the presentinvention may include a photoelectric element part (91), a firstadhesive film (92(a)) formed on the photoelectric element part (91), atouch function part (93) formed on the first adhesive film (92(a)), asecond adhesive film (92(b)) formed on the touch function part (93) anda window film (95) formed on the second adhesive film (92(b)).

In some embodiments, the first adhesive film (92(a)) or the secondadhesive film (92(b)) may have an average slope of about −9.9 to about 0at −20° C. to 80° C. based on x-axis for the distribution of storagemodulus versus temperature in the function of x-axis of temperature (°C.) and y-axis of storage modulus (KPa). The average slope may bespecifically from about −9.9, −9.5, −9, −8.5, −8, −7.5, −7, −6.5, −6,−5.5, −5, −4.5, −4, −3.5, −3, −2.5, −2, −1.5, −1, −0.5 or 0. Inaddition, the first adhesive film (92(a)) or the second adhesive film(92(b)) may have equal, more or less average slope of among the abovementioned examples. The average slope may be specifically about −5 toabout 0, more specifically about −2 to about 0. Within this range, theadhesive film may exhibit viscoelasticity property at a wide temperaturerange, and have excellent recovery rate, and thus can be used for theflexible display device.

The average slope means an average slope at −20° C. to 80° C. based onx-axis for the distribution of storage modulus versus temperature of theadhesive film, in which a temperature (° C.) is plotted as x-axis and astorage modulus (KPa) is plotted as y-axis, and is calculated accordingto Equation 3:

Average slope=(Mo(80° C.)−Mo(−20° C.))/(80−(−20))  [Equation 3]

(Where in equation 3, Mo (80° C.) is a storage modulus at 80° C., and Mo(−20° C.) is a storage modulus at −20° C.)

The first adhesive film (92(a)) or the second adhesive film (92(b)) mayhave a storage modulus of about 10 KPa or more at 80° C. For example,the first adhesive film (92(a)) or the second adhesive film (92(b)) maybe about 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150, 200, 250, 300,350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950 or 1000Kpa. In addition, the first adhesive film (92(a)) or the second adhesivefilm (92(b)) may have equal, more or less storage modulus at 80° C. ofamong the above mentioned examples. Specifically, the first adhesivefilm (92(a)) or the second adhesive film (92(b)) may have a about 10 KPato about 1000 KPa, specifically about 10 KPa to about 800 KPa, and morespecifically about 10 KPa to about 100 KPa at 80° C. Within this range,the adhesive film may have excellent viscoelasticity (elastic) propertyand recovery rate at a high temperature.

To improve peel strength of the first adhesive film (92(a)) or thesecond adhesive film (92(b)), a surface onto which the adhesivecomposition is coated may be subjected to surface treatment in advance,for example, corona pretreatment at about 150 mJ/cm². For example,corona pretreatment may be performed by treating a surface of anadherend (for example, a PET film) twice under corona discharge at adose of about 78 using a corona treatment device (Now plasma Co., Ltd.),without being limited thereto.

The first adhesive film (92(a)) or the second adhesive film (92(b)) mayhave a T-peel strength of about 400 gf/in to about 5,000 gf/in atthickness of about 100 μm, as measured at room temperature (about 25°C.) with respect to a corona-treated polyethylene terephthalate (PET)film. For example, the first adhesive film or the second adhesive filmmay have a T-peel strength of about 400, 500, 600, 700, 800, 900, 1000,1100, 1200, 1300, 1400, 1500, 1600, 1700, 1800, 1900, 2000, 2100, 2200,2300, 2400, 2500, 2600, 2700, 2800, 2900, 3000, 3100, 3200, 3300, 3400,3500, 3600, 3700, 3800, 3900, 4000, 4100, 4200, 4300, 4400, 4500, 4600,4700, 4800, 4900 or 5000 gf/in. In addition, the first adhesive film(92(a)) or the second adhesive film (92(b)) may have a T-peel strengthmay have equal, more or less of among the above mentioned examples.Specifically, the first adhesive film (92(a)) or the second adhesivefilm (92(b)) may have a T-peel strength of about 500 gf/in to about4,000 gf/in, more specifically about 700 gf/in to about 3,500 gf/in.Within this range, the first adhesive film or the second adhesive filmmay have excellent adhesion and reliability at a high temperature.

The first adhesive film (92(a)) or the second adhesive film (92(b)) mayhave a T-peel strength of about 200 gf/in to about 3,000 gf/in, asmeasured at 60° C. at thickness of 100 μm with respect to acorona-treated polyethylene terephthalate (PET) film. For example, thefirst adhesive film or the second adhesive film may have a T-peelstrength of about 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100,1200, 1300, 1400, 1500, 1600, 1700, 1800, 1900, 2000, 2100, 2200, 2300,2400, 2500, 2600, 2700, 2800, 2900 or 3000 gf/in at thickness of 100 μmwith respect to a corona-treated polyethylene terephthalate (PET) filmat 60° C. In addition, the T-peel strength may have equal, more or lessof among the above mentioned examples. Specifically, the first adhesivefilm or the second adhesive film may have a T-peel strength of about 500gf/in to about 2,000 gf/in, more specifically about 500 gf/in to about1,500 gf/in at thickness of 100 μm. Within this range, the firstadhesive film or the second adhesive film may have excellent adhesionand reliability at a high temperature.

The T-peel strength of the adhesive film is measured as follows. Aspecimen is prepared by laminating corona-pretreated surfaces of PETfilms having a size of about 150 mm×about 25 mm×about 75 μm(length×width×thickness) to both surfaces of the adhesive film having asize of about 100 mm×about 25 mm×about 100 μm (length×width×thickness).Next, the specimen is subjected to autoclaving under conditions of about3.5 bar and about 50° C. for about 1,000 seconds and then secured to aTA.XT_Plus texture analyzer (Stable Micro System Co., Ltd.). At 25° C.or 60° C., the PET film at one side is kept fixed and the PET film atthe other side is pulled at a rate of about 50 mm/min, thereby measuringT-peel strength of the adhesive film with respect to the PET film.Corona pretreatment of the PET film may be performed, for example, bytreating the PET film twice (total dose: about 156) under coronadischarge at a dose of about 78 using a corona treatment device (Nowplasma Co., Ltd.).

The first adhesive film (92(a)) or the second adhesive film (92(b)) mayhave a storage modulus of about 10 KPa or more, for example, about 10KPa to about 1000 KPa, specifically about 10 KPa to about 800 KPa, andmore specifically about 10 KPa to about 100 KPa at 80° C. Within thisrange, the adhesive film may have excellent viscoelasticity (elastic)property and recovery rate at a high temperature.

The first adhesive film (92(a)) or the second adhesive film (92(b)) mayinclude an adhesive film having a storage modulus of about 10 KPa toabout 1000 KPa, specifically about 10 KPa to about 800 KPa, and morespecifically about 20 KPa to about 500 KPa at 25° C. Within this range,the adhesive film may have excellent viscoelasticity property andrecovery rate at an ambient temperature.

The first adhesive film (92(a)) or the second adhesive film (92(b)) mayinclude an adhesive film having a storage modulus of about 10 KPa toabout 1000 KPa, specifically about 10 KPa to about 500 KPa, and morespecifically about 20 KPa to about 500 KPa at −20° C. Within this range,when these are used in the flexible device at a low temperature, theadhesive film is flexible and has no whitening event, and thus can beused for the optical elements.

The first adhesive film (92(a)) or the second adhesive film (92(b)) mayhave a ratio of a storage modulus at 80° C. to a storage modulus at −20°C. of about 1:1 to about 1:10, for example, specifically about 1:1 toabout 1:8, more specifically about 1:1 to about 1:5. Within this range,the adhesive film may have excellent adhesion between the adhesive andthe adherent at a wide temperature range (−20° C. to 80° C.), and can beused for the flexible display device.

The first adhesive film (92(a)) or the second adhesive film (92(b)) mayinclude an adhesive film having a haze of about 5% or less, specificallyabout 3% or less, and more specifically about 1% or less at a thicknessof about 100 μm. Within this range, when the adhesive film is used inthe display, it may exhibit excellent transparency.

The first adhesive film (92(a)) or the second adhesive film (92(b)) mayhave a haze of about 5% or less, specifically 3% or less, and morespecifically 1% or less at a thickness of 100 μm after 200% stretching.Within this range, when the adhesive film is used in the display, it mayexhibit excellent transparency.

The first adhesive film (92(a)) or the second adhesive film (92(b)) mayinclude an adhesive film having a recovery rate from about 30% to about98% at a thickness of about 100 μm. For example, the first adhesive film(92(a)) or the second adhesive film (92(b)) may include an adhesive filmhaving a recovery rate from about 30, 31, 32, 33, 34, 35, 36, 37, 38,39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56,57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74,75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92,93, 94, 95, 96, 97 or 98% at a thickness of about 100 μm. In addition,the first adhesive film (92(a)) or the second adhesive film (92(b)) mayinclude an adhesive film having a recovery rate of equal, more or lessof among the above mentioned examples. Specifically, the recovery ratemay be about 40% to about 95% at a thickness of 100 μm, as measuredaccording to Equation 1. Within this range, the adhesive film may beused for the flexible display device, and have long shelf life.

Recovery rate (%)=(1−(X _(f) /X ₀))×100  [Equation 1]

(where in equation 1, when both ends of each of polyethyleneterephthalate (PET) films (thickness: about 75 μm) are defined as afirst end and a second end, respectively, a specimen is prepared bybonding ends of two PET films to each other via an adhesive film havinga size of 20 mm×20 mm (length×width) in order of first end of first PETfilm/adhesive film (length×width: 20 mm×20 mm)/second end of second PETfilm. Next, jigs are secured to non-bonded ends of the PET films of thespecimen, respectively. Next, the jig at one side is kept fixed and thejig at the other side is pulled to a length (about 10 times ofthickness, X₀) of about 1,000% of thickness (unit: μm) of the adhesivefilm of the adhesive film at a rate of about 300 mm/min and thenmaintained for about 10 seconds. When a force of about 0 kPa is appliedto the adhesive film by recovering the adhesive film at the same rate(300 mm/min) as the pulling rate, an increased length of the adhesivefilm is defined as X_(f) (unit: μm)).

The first adhesive film (92(a)) or the second adhesive film (92(b)) mayhave an elongation of about 800% to about 2000% as measured by rollingclosely the adhesive film of a size of about 5 cm×about 5 cm and athickness of about 100 μm, and fixing it to the both ends of a TA (TA.XTPlus Texture Analyzer (Stable Micro Systems, Ltd)), and then stretchingit at a rate of about 300 mm/min. Specifically, the adhesive film mayhave a ratio of the length at time which the adhesive film breaks to thelength prior to stretching (elongation) of about 800% to about 2000%,specifically about 800% to about 1800%, and more specifically about 900%to about 1700%. Within this range, the adhesive film may be used for theflexible display device, and have excellent reliability.

The first adhesive film (92(a)) or the second adhesive film (92(b)) mayhave a bubble generation area of about 0%, as measured when the adhesivefilm of about 13 cm×about 3 cm, thickness 100 μm, comprising about 50 μmthick PET film stacked on one surface thereof and a about 100 μm thickPET film stacked on the other surface thereof is bent towards the about50 μm thick PET film such that the adhesive film has about ½ of thelength, followed by placing the adhesive film between parallel frameshaving a gap of about 1 cm, and then subjected to aging under conditionsof about 70° C. and humidity of about 93% for about 24 hours. Withinthis range, the adhesive film may be used for the flexible displaydevice, and have excellent reliability.

The first adhesive film (92(a)) or the second adhesive film (92(b)) inthe flexible display device (100) may have a bubble generation area of0%, as measured when the flexible display device that a window film wasreplaced with PET film (thickness of 100 μm), bended into the parallelframes having a spacing of 1 cm in the direction of the photoelectricelement part, and aging at temperature of 70° C. and humidity of 93% for24 hours. Within this range, the bubble may spread, diffuse, transfer orthe like, then the bubble generation area reduced, thus the adhesivefilm and flexible display device have excellent reliability.

The first adhesive film (92(a)) or the second adhesive film (92(b)) mayhave a thickness of 1 μm to 2 mm, and specifically 50 μm to 1 mm. Withinthis range, the adhesive film may be used for the optical displaydevice.

The first adhesive film (92(a)) or the second adhesive film (92(b)) maybe formed by curing the pressure-sensitive adhesive composition. Inother words, the first adhesive film (92(a)) or the second adhesive film(92(b)) may include a cured product of the pressure-sensitive adhesivecomposition.

The pressure-sensitive adhesive composition may include a (meth)acryliccopolymer having a hydroxyl group formed of monomer mixture, and organicparticles.

The monomer mixture may include the (meth)acrylate monomer having thehydroxyl group and comonomer. And the (meth)acrylic copolymer having thehydroxyl group may be polymerized from a monomer mixture including the(meth)acrylate monomer having the hydroxyl group and comonomer.

The (meth)acrylate monomer having the hydroxyl group may be a(meth)acrylic ester having a C₁ to C₂₀ alkyl group having one or morehydroxyl groups, a (meth)acrylic ester having a C₅ to C₂₀ cycloalkylgroup having one or more hydroxyl groups, or a (meth)acrylic esterhaving a C₆ to C₂₀ aryl group having one or more hydroxyl groups.

The (meth)acrylate monomer having the hydroxyl group may be, forexample, but not limited thereto, at least one of 2-hydroxyethyl(meth)acrylate, 4-hydroxybutyl (meth)acrylate, 2-hydroxypropyl(meth)acrylate, 2-hydroxybutyl (meth)acrylate, or 6-hydroxyhexyl(meth)acrylate. Particularly, it is possible to further increase theadhesion by using a (meth)acrylic monomer having a C₁ to C₅ alkyl grouphaving one or more hydroxyl groups.

The (meth)acrylic monomer having the hydroxyl group may be present in anamount of about 5 wt % to about 40 wt %, for example about 10 wt % toabout 30 wt % in the monomer mixture. Within this range, the adhesivefilm may have reliability, and excellent adhesion.

The comonomer may be, for example, but not limited thereto, at least oneof an alkyl (meth)acrylate monomer, a monomer having ethylene oxide, amonomer having propylene oxide, a monomer having amine group, a monomerhaving amide group, a monomer having alkoxy group, a monomer havingphosphate group, a monomer having sulfonic acid group, a monomer havingphenyl group and a monomer having silane group.

The alkyl (meth)acrylate monomer may include an unsubstituted linear orbranched C₁ to C₂₀ alkyl (meth)acrylic ester. For example, the alkyl(meth)acrylate monomer may include at least one of methyl(meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-butyl(meth)acrylate, t-butyl (meth)acrylate, iso-butyl (meth)acrylate, pentyl(meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, ethylhexyl(meth)acrylate, octyl (meth)acrylate, isooctyl (meth)acrylate, nonyl(meth)acrylate, decyl (meth)acrylate, or lauryl (meth)acrylate.Preferably, it is possible to further increase the initial adhesion byusing a C₄ to C₈ alkyl (meth)acrylic monomer.

The monomer having ethylene oxide may include at least one of(meth)acrylate monomer containing ethylene oxide group (—CH₂CH₂O—). Forexample, the monomer may be, but not limited thereto, polyethylene oxidealkyl ether (meth)acrylate such as polyethylene oxide monomethyl ether(meth)acrylate, polyethylene oxide monoethyl ether (meth)acrylate,polyethylene oxide monopropyl ether (meth)acrylate, polyethylene oxidemonobutyl ether (meth)acrylate, polyethylene oxide monopentyl ether(meth)acrylate, polyethylene oxide dimethyl ether (meth)acrylate,polyethylene oxide diethyl ether (meth)acrylate, polyethylene oxidemonoisopropyl ether (meth)acrylate, polyethylene oxide monoisobutylether (meth)acrylate, polyethylene oxide mono tert-butyl ether(meth)acrylate, and the like.

The monomer having propylene oxide may be, but not limited thereto, apolypropylene oxide alkylether (meth)acrylate such as polypropyleneoxide monomethyl ether (meth)acrylate, polypropylene oxide monoethylether (meth)acrylate, polypropylene oxide monopropyl ether(meth)acrylate, polypropylene oxide monobutyl ether (meth)acrylate,polypropylene oxide monopentyl ether (meth)acrylate, polypropylene oxidedimethyl ether (meth)acrylate, polypropylene oxide diethyl ether(meth)acrylate, polypropylene oxide monoisopropyl ether (meth)acrylate,polypropylene oxide monoisobutyl ether (meth)acrylate, polypropyleneoxide mono tert-butyl ether (meth)acrylate, and the like.

The monomer having amino group may be, but not limited thereto, an aminogroup containing (meth)acrylic monomer such as monomethylaminoethyl(meth)acrylate, monoethylaminoethyl (meth)acrylate,monomethylaminopropyl (meth)acrylate, monoethylaminopropyl(meth)acrylate, dimethylaminoethyl (meth)acrylate, diethylaminoethyl(meth)acrylate, N-tert-butylaminoethyl (meth)acrylate,methacryloxyethyltrimethylammonium chloride (meth)acrylate, and thelike.

The monomer having amide group may be, but not limited thereto, an amidegroup containing (meth)acrylic monomer such as (meth)acryl amide,N-methylacryl amide, N-methylmethacryl amide, N-methylol (meth)acrylamide, N-methoxymethyl (meth)acryl amide, N,N-methylene bis (meth)acrylamide, 2-hydroxyethylacryl amide, and the like.

The monomer having alkoxy group may be, but not limited thereto,2-methoxy ethyl (meth)acrylate, 2-methoxypropyl (meth)acrylate,2-ethoxypropyl (meth)acrylate, 2-buthoxypropyl (meth)acrylate,2-methoxypentyl (meth)acrylate, 2-ethoxypentyl (meth)acrylate,2-buthoxyhexyl (meth)acrylate, 3-methoxypentyl (meth)acrylate,3-ethoxypentyl (meth)acrylate, or 3-buthoxyhexyl (meth)acrylate.

The monomer having phosphate group may be, but not limited thereto, aphosphate group containing acrylic monomer such as2-methacryloyloxyethyldiphenyl phosphate (meth)acrylate,trimethacryloyloxyethyl phosphate (meth)acrylate, triacryloyloxyethylphosphate (meth)acrylate, and the like.

The monomer having sulfonic acid group may be, but not limited thereto,a sulfonic acid group containing acrylic monomer such as sulfopropyl(meth)acrylate sodium, 2-sulfoethyl (meth)acrylate sodium,2-acrylamido-2-methylpropane sulfonic acid sodium, and the like.

The monomer having phenyl group may be, but not limited thereto, aphenyl group containing acrylic vinyl monomer such as p-tert-butylphenyl (meth)acrylate, o-biphenyl (meth)acrylate, and the like.

The monomer having silane group may be, but not limited thereto, asilane group containing vinyl monomer such as 2-acetoacetoxyethyl(meth)acrylate, vinyltrimethoxy silane, vinyltriethoxy silane, vinyltris (β-methoxyethyl) silane, vinyltriacetyl silane,methacryloyloxypropyltrimethoxy silane, and the like.

The comonomer may be present in an amount of about 60 wt % to about 95wt %, for example about 70 wt % to about 90 wt % in the monomer mixture.Within this range, the adhesive film may have excellent adhesion andreliability.

In another embodiment, the comonomer having a glass transitiontemperature (Tg) of −150° C. to 0° C. may be used. The glass transitiontemperature may be determined, for example, on a homopolymer of eachmonomer to be measured using a DSC Discovery from TA Instruments.Specifically, the glass transition temperature may be determined on thehomopolymer of each monomer by increasing the temperature to about 180°C. at a rate of 20° C./min and allowing it cool slowly to −80° C.,thereby the data of an endothermic transition curve at time which thetemperature increases to 180° C. at a rate of 10° C./min, and the pointof inflection of the endothermic transition curve may be taken as theglass transition temperature. The comonomer having a glass transitiontemperature (Tg) of −150° C. to 0° C. may be used without limitation aslong as it has a glass transition temperature (Tg) of −150° C. to 0° C.Specifically, the monomer having a glass transition temperature (Tg) of−150° C. to −20° C., more specifically the monomer having a glasstransition temperature (Tg) of −150° C. to −40° C. may be used.

In still another embodiment, the comonomer having a glass transitiontemperature (Tg) of −150° C. to 0° C., selected from an alkyl(meth)acrylate monomer, a monomer having ethylene oxide, a monomerhaving propylene oxide, a monomer having amine group, a monomer havingamide group, a monomer having alkoxy group, a monomer having phosphategroup, a monomer having sulfonic acid group, a monomer having phenylgroup and a monomer having silane group may be used.

The comonomer may be, for example, at least one of an alkyl(meth)acrylate monomer such as methyl acrylate, ethyl acrylate, isopropyl acrylate, n-butyl acrylate, iso-butyl acrylate, hexyl(meth)acrylate, heptyl (meth)acrylate, 2-ethylhexyl acrylate, dodecyl(meth)acrylate, and the like; an alkylene oxide group containing(meth)acrylate monomer such as polyethylene oxide monomethyl ether(meth)acrylate, polyethylene oxide monoethyl ether (meth)acrylate,polyethylene oxide monopropyl ether (meth)acrylate, polyethylene oxidemonobutyl ether (meth)acrylate, polyethylene oxide monopentyl ether(meth)acrylate, polypropylene oxide monomethyl ether (meth)acrylate,polypropylene oxide monoethyl ether (meth)acrylate, polypropylene oxidemonopropyl ether (meth)acrylate, and the like; an amino group containing(meth)acrylate monomer such as monomethylaminoethyl (meth)acrylate,monoethylaminoethyl (meth)acrylate, monomethylaminopropyl(meth)acrylate, monoethylamino propyl (meth)acrylate, and the like; analkoxy group containing (meth)acrylate monomer such as 2-methoxy ethyl(meth)acrylate, 2-methoxypropyl (meth)acrylate, 2-ethoxypropyl(meth)acrylate, 2-acetoacetoxyethyl (meth)acrylate, and the like; and asilane group containing (meth)acrylate monomer such as vinyltrimethoxysilane, vinyltriethoxy silane, and the like.

In some embodiments, the monomer mixture having may includes the(meth)acrylate monomer having the hydroxyl group and the monomer havinga glass transition temperature (Tg) of −150° C. to 0° C. In this case,the monomer having a glass transition temperature (Tg) of −150° C. to 0°C. may be present in an amount of about 60 wt % to about 95 wt %, andfor example, about 70 wt % to about 90 wt % in the monomer mixture.Within this range, the adhesive film may have excellent adhesion andreliability. The (meth)acrylic monomer having the hydroxyl group may bepresent in an amount of about 5 wt % to about 40 wt %, for example about10 wt % to about 30 wt % in the monomer mixture. Within this range, theadhesive film may have low haze, and excellent adhesion.

In some embodiments, the monomer mixture may further include a monomerhaving a carboxy group.

The monomer having the carboxy group may be, for example, but notlimited thereto, (meth)acrylic acid, 2-carboxyethyl (meth)acrylate,3-carboxypropyl (meth)acrylate, 4-carboxybutyl (meth)acrylate, itaconicacid, crotonic acid, maleic acid, fumaric acid and anhydrous maleicacid, and the like.

For example, the monomer having the carboxy group may be further presentin an amount of about 10 wt % or less, and in some embodiments about 7wt % or less, specifically about 5 wt % or less, and more specifically 1wt % or less in the monomer mixture. Within this range, the adhesivefilm may have high adhesion, and excellent reliability.

Since the pressure-sensitive adhesive composition or the adhesive filmincludes the organic particles, it has excellent viscoelasticity at alow temperature and/or at an room ambient temperature and crosslinkedstructure, and thus exhibits viscoelasticity stably at a hightemperature. In some embodiments, the organic particles may form achemical bond with the (meth)acrylic copolymer having the hydroxylgroup.

In one embodiment, even though the pressure-sensitive adhesivecomposition or the adhesive film includes the organic particles, it hasa certain refractive index difference with the organic particles havinga certain average particle size described below and the (meth)acryliccopolymer having the hydroxyl group, thereby having excellenttransparency.

The organic particles may have an average particle diameter of about 10nm to about 400 nm, for example, about 10, 20, 30, 40, 50, 60, 70, 80,90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220,230, 240, 250, 260, 270, 280, 290, 300, 310, 320, 330, 340, 350, 360,370, 380, 390 or 400. In addition, The organic particles may have anaverage particle diameter of equal, more or less of among the the abovementioned examples. Specifically, the organic particles may have anaverage particle diameter of 10 nm to 300 nm, and more specifically 10nm to 200 nm. Within this range, it is possible to prevent aggregationof the organic particles, and achieve excellent transparency.

The organic particles may have a refractive index difference with the(meth)acrylic copolymer having the hydroxyl group of about 0.05 or less,specifically about 0 or more and about 0.03 or less, and specificallyabout 0 or more and about 0.02 or less. Within this range, the adhesivefilm may have excellent transparency.

The organic particles may have core-shell structures, and the glasstransition temperature of the core and the shell satisfies the followingEquation 2:

Tg(c)<Tg(s)  [Equation 2]

(wherein, Tg(c) is a glass transition temperature (° C.) of the core,and Tg(s) is a glass transition temperature (° C.) of the shell.)

Specifically, the core may have a glass transition temperature of about−150° C. to about 10° C., specifically about −150° C. to about −5° C.,and more specifically about −150° C. to about −20° C. Within this range,it is possible to achieve a storage modulus value required at a lowtemperature (−20° C.), and excellent viscoelasticity property at a lowtemperature and/or at an ambient temperature.

Specifically, the core may include at least one polyalkyl (meth)acrylatehaving a glass transition temperature described above. For example, thecore may include, but not limited thereto, at least one of polymethylacrylate, polyethyl acrylate, polypropyl acrylate, polybutyl acrylate,polyisopropyl acrylate, polyhexyl acrylate, polyhexyl methacrylate,polyethylhexyl acrylate and polyethylhexyl methacrylate. Preferably, thecore may include at least one of polybutyl acrylate and polyethylhexylacrylate.

The shell may have a glass transition temperature of about 15° C. toabout 150° C., specifically about 35° C. to about 150° C., and morespecifically about 50° C. to about 140° C. Within this range, it mayhave excellent dispersibility of the organic particle in the(meth)acrylic copolymer having the hydroxyl group.

Specifically, the shell may include a polyalkyl (meth)acrylate having aglass transition temperature described above. For example, the shell mayinclude, but not limited thereto, at least one of polymethylmethacrylate (PMMA), polyethyl methacrylate, polypropyl methacrylate,polybutyl methacrylate, polyisopropyl methacrylate, polyisobutylmethacrylate and polycyclohexyl methacrylate. For example, the shell mayinclude polymethyl methacrylate.

In another embodiment, the core or the shell may include two or more oflayers, and the outermost layer of the organic particle may include atleast one polyalkyl (meth)acrylate having a glass transition temperatureof about 15° C. to about 150° C. Specifically, the core may include atleast one polyalkyl (meth)acrylate having a glass transition temperatureof about −150° C. to about 10° C., or alternatively include at least onepolyalkyl (meth)acrylate without limitation in the glass transitiontemperature, but satisfying the glass transition temperature of the coreto about −150° C. to about 10° C., but not limited thereto. In addition,the shell may include at least one polyalkyl (meth)acrylate having aglass transition temperature of about 15° C. to about 150° C., oralternatively include at least one polyalkyl (meth)acrylate withoutlimitation in the glass transition temperature, but satisfying the glasstransition temperature of the shell to about 15° C. to about 150° C.,but not limited thereto.

The shell may be present in an amount of about 1 wt % to about 70 wt %,specifically about 5 wt % to about 60 wt %, and more specifically about10 wt % to about 50 wt % in the organic particle. Within this range, itis possible to maintain viscoelasticity property at a wide temperaturerange, and achieve excellent recovery rate of the adhesive film.

In some embodiments, the organic particle may be present in an amount ofabout 0.1 parts by weight to about 15 parts by weight, specificallyabout 0.1 parts by weight to about 10 parts by weight, about 0.5 partsby weight to about 10 parts by weight, and more specifically about 0.1parts by weight to about 8 parts by weight based on 100 parts by weightof the (meth)acrylic copolymer having the hydroxyl group. Within thisrange, it is possible to achieve a balance between viscoelasticity,storage modulus and recovery rate.

In one embodiment, the (meth)acrylic copolymer having the hydroxyl groupmay include a monomer mixture including a comonomer (for example, acomonomer having a glass transition temperature (Tg) from about −150° C.to about 0° C.) of about 60 wt % to about 95 wt %, and for example,about 70 wt % about 90 wt %, and a (meth)acrylic monomer having ahydroxyl group of about 5 wt % to about 40 wt %, for example, about 10wt % to about 30 wt %. Within this range, the adhesive film may haveexcellent adhesion and reliability.

In another embodiment, the (meth)acrylic copolymer having the hydroxylgroup may include a comonomer (for example, a comonomer having a glasstransition temperature (Tg) of −150° C. to 0° C.) of about 60 wt % toabout 95 wt %, for example, about 70 wt % to about 90 wt %, and a(meth)acrylic monomer having a hydroxyl group of about 5 wt % to about40 wt %, for example, about 10 wt % to about 30 wt %, and the monomerhaving the carboxyl group may include the (meth)acrylic copolymer havingthe hydroxyl group of about 0 wt % to about 10 wt % or less, about 0.1wt % to about 1 wt % in the monomer mixture. Within this range, thepressure-sensitive adhesive composition may have excellent adhesion andreliability.

In one embodiment, the organic particle in the pressure-sensitiveadhesive composition may be used in the polymerized form with themonomer mixture when preparing the (meth)acrylic copolymer having thehydroxyl group. In this case, the organic particle may be used in theform included in the (meth)acrylic copolymer having the hydroxyl group.

In another embodiment, the pressure-sensitive adhesive composition mayinclude the organic particle in combination with the already prepared(meth)acrylic copolymer having the hydroxyl group. In this case, thepressure-sensitive adhesive composition may include the organic particlein the different state from the (meth)acrylic copolymer having thehydroxyl group.

The (meth)acrylic copolymer having the hydroxyl group may have a glasstransition temperature of about −150° C. to about −13° C., andspecifically about −100° C. to about −20° C. Within this range, theadhesive film may have excellent foldability, as well as an excellentadhesion and reliability at a wide temperature range.

The (meth)acrylic copolymer having the hydroxyl group may be preparedspecifically by mixing the monomer mixture and the organic particleswith a radical photo-polymerizable initiator, followed by a solutionpolymerization, a suspension polymerization, a photo polymerization, abulk polymerization, a dispersion polymerization or an emulsionpolymerization. Alternatively, The (meth)acrylic copolymer having thehydroxyl group may be prepared by partially polymerizing the monomermixture to obtain a prepolymer, and adding the organic particles intothe prepolymer. Specifically, emulsion polymerization may be performedby adding an initiator to a dispersant, a crosslinking agent, themonomer mixture and the organic particles in an aqueous solution at 25°C. to 100° C.

In some embodiments, the pressure-sensitive adhesive composition mayfurther include at least one of a crosslinking agent and an initiator.

The crosslinking agent may be, but not limited thereto, as apolyfunctional (meth)acrylate, for example a bifunctional acrylate suchas 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate,neopentylglycol di(meth)acrylate, polyethyleneglycol di(meth)acrylate,neopentylglycol adipate di(meth)acrylate, dicyclopentanyldi(meth)acrylate, caprolactone modified dicyclopentenyldi(meth)acrylate, ethylene oxide modified di(meth)acrylate, di(meth)acryloxyethyl isocyanurate, allylated cyclohexyl di(meth)acrylate,tricyclodecane dimethanol (meth)acrylate, dimethylol dicyclopentanedi(meth)acrylate, ethylene oxide modified hexahydrophthalic aciddi(meth)acrylate, tricyclodecane dimethanol (meth)acrylate,neopentylglycol modified trimethylpropane di(meth)acrylate, adamantanedi(meth)acrylate or 9,9-bis[4-(2-acryloyloxyethoxy)phenyl]fluorene, andthe like; a trifunctional acrylate such as trimethylolpropanetri(meth)acrylate, dipentaerythritol tri(meth)acrylate, propionic acidmodified dipentaerythritol tri(meth)acrylate, pentaerythritoltri(meth)acrylate, propyleneoxide modified trimethylolpropanetri(meth)acrylate, trifunctional urethane (meth)acrylate or tris(meth)acryloxyethyl isocyanurate, and the like; a tetrafunctionalacrylate such as diglycerine tetra(meth)acrylate or pentaerythritoltetra(meth)acrylate, and the like; a pentafunctional acrylate such asdipentaerythritol penta (meth)acrylate, and the like; and ahexafunctional acrylate such as dipentaerythritol hexa(meth)acrylate,caprolactone modified dipentaerythritol hexa(meth)acrylate or urethane(meth)acrylate (ex. hexa(meth)acrylate reactant of isocyanate monomerand trimethylolpropane tri(meth)acrylate), for example, reactant ofisocyanate monomer and trimethylolpropane tri(meth)acrylate, and thelike. These may be use alone or mixtures thereof. Preferably, thecrosslinking agent may have excellent durability and reliability byusing a polyfunctional (meth)acrylate of a polyhydric alcohol having2-20 hydroxyl groups.

The crosslinking agent may be present in an amount of about 0.01 part byweight to about 10 parts by weight, specifically about 0.03 parts byweight to about 7 parts by weight, and specifically about 0.1 part byweight to about 5 parts by weight based on 100 parts by weight of the(meth)acrylic copolymer having the hydroxyl group. Within this range,the adhesive film may have excellent durability and an increase inreliability.

The initiator may be a photopolymerization initiator or a thermalpolymerization initiator.

The initiator may be an initiator which is the same as or different fromthe initiator used in the preparation of the hydroxyl group-containing(meth)acrylic copolymer. In another embodiment, the initiator may be athermal polymerization initiator.

The photopolymerization initiator may be any initiator so long as theinitiator can realize a second crosslinking structure by derivingpolymerization of the radical polymerizable compound during curingthrough light irradiation. For example, the photopolymerizationinitiator may include benzoin, hydroxyl ketone, amino ketone, phosphineoxide photoinitiators, and the like. Specifically, thephotopolymerization initiator may include benzoin, benzoin methyl ether,benzoin ethyl ether, benzoin isopropyl ether, benzoin n-butyl ether,benzoin isobutyl ether, acetophenone, dimethylamino acetophenone,2,2-dimethoxy-2-phenylacetophenone, 2,2-diethoxy-2-phenylacetophenone,2-hydroxy-2-methyl-1-phenylpropan-1-one, 1-hydroxycyclohexyl phenylketone, 2-methyl-1-[4-(methylthio)phenyl]-2-morpholino-propan-1-one,4-(2-hydroxyethoxy)phenyl-2-(hydroxy-2-propyl)ketone, benzophenone,p-phenylbenzophenone, 4,4′-bis(diethyl)aminobenzophenone,dichlorobenzophenone, 2-methylanthraquinone, 2-ethylanthraquinone,2-t-butylanthraquinone, 2-aminoanthraquinone, 2-methylthioxanthone,2-ethylthioxanthone, 2-chlorothioxanthone, 2,4-dimethylthioxanthone,2,4-diethylthioxanthone, benzyl dimethyl ketal, acetophenone dimethylketal, p-dimethylaminobenzoic acid ester,oligo[2-hydroxy-2-methyl-1-[4-(1-methylvinyl)phenyl]propanone], and2,4,6-trimethylbenzoyl-diphenyl-phosphine oxide, without being limitedthereto. These photopolymerization initiators may be used alone or incombination thereof.

The thermal polymerization initiator may be any initiator withoutlimitation so long as the initiator can realize a second crosslinkingstructure by deriving polymerization. For example, the thermalpolymerization initiator may include typical initiators such as azo,peroxide and redox compounds. Examples of the azo compound may include2,2-azobis(2-methylbutyronitrile), 2,2-azobis(isobutyronitrile),2,2-azobis(2,4-dimethylvaleronitrile),2,2-azobis-2-hydroxymethylpropionitrile,dimethyl-2,2-methylazobis(2-methylpropionate), and2,2-azobis(4-methoxy-2,4-dimethylvaleronitrile) without being limitedthereto. Examples of the peroxide compound may include: inorganicperoxides such as potassium perchlorate, ammonium persulfate andhydrogen peroxide; and organic peroxides such as diacyl peroxide,peroxydicarbonate, peroxyester, tetramethylbutyl peroxyneodecanoate,bis(4-butylcyclohexyl) peroxydicarbonate, di(2-ethylhexyl)peroxycarbonate, butyl peroxyneodecanoate, dipropyl peroxydicarbonate,diisopropyl peroxydicarbonate, diethoxyethyl peroxydicarbonate,diethoxyhexyl peroxydicarbonate, hexyl peroxydicarbonate, dimethoxybutylperoxydicarbonate, bis(3-methoxy-3-methoxybutyl) peroxydicarbonate,dibutyl peroxydicarbonate, dicetyl peroxydicarbonate, dimyristylperoxydicarbonate, 1,1,3,3-tetramethylbutyl peroxypivalate, hexylperoxypivalate, butyl peroxypivalate, trimethylhexanoyl peroxide,dimethyl hydroxybutyl peroxyneodecanoate, amyl peroxyneodecanoate,t-butyl peroxy neoheptanoate, amyl peroxypivalate, t-butylperoxypivalate, t-amyl peroxy-2-ethylhexanoate, lauroyl peroxide,dilauroyl peroxide, di(dodecanoyl) peroxide, benzoyl peroxide, anddibenzoyl peroxide, without being limited thereto. Examples of the redoxcompound may include mixtures of a peroxide compound and a reductant,without being limited thereto. These azo, peroxide and redox compoundsmay be used alone or in combination thereof.

The initiator may be present in an amount of about 0.01 parts by weightto about 5 parts by weight, specifically about 0.05 parts by weight toabout 3 parts by weight, more specifically about 0.1 parts by weight toabout 1 part by weight, based on 100 parts by weight of the monomermixture forming the hydroxyl group-containing (meth)acrylic copolymer.Within this range, curing can be completely performed, deterioration intransmittance of the adhesive composition due to the residual initiatorcan be prevented, bubble generation in the adhesive composition can bereduced, and the adhesive composition can have excellent reactivity.

In another embodiment, the pressure-sensitive adhesive composition mayfurther include a silane coupling agent.

The silane coupling agent may include, but not limited thereto, siloxaneor epoxy silane coupling agents. The silane coupling agent may bepresent in an amount of about 0.01 part by weight to about 0.1 part byweight, preferably about 0.05 parts by weight to about 0.1 part byweight based on 100 parts by weight of the (meth)acrylic copolymerhaving the hydroxyl group. Within this range, it is possible to increasethe reliability.

The pressure-sensitive adhesive composition may optionally furtherinclude a typical additives such as hardening accelerators, ionicliquids, lithium salts, inorganic fillers, softening agents, molecularweight regulators, antioxidants, anti-aging agents, stabilizers,tackifier resins, modified resins (ex. polyol resins, phenol resins,acryl resins, polyester resins, polyolefin resins, epoxy resins,epoxidated polybutadiene resins, etc.), leveling agents, anti-foamingagents, plasticizers, dyes, pigments (ex. coloring pigments, extenderpigments, etc.), treating agents, UV blocking agents, fluorescentwhitening agents, dispersants, thermal stabilizers, photostabilizers, UVabsorbing agents, antistatic agents, coagulants, lubricants andsolvents, and the like.

The pressure-sensitive adhesive composition may further include anon-cured compound.

The pressure-sensitive adhesive composition may have a viscosity ofabout 300 cPs to about 50,000 cPs at 25° C. Within this range, thepressure-sensitive adhesive composition may have excellent coatabilityand uniformity of the thickness.

According to the FIG. 1, the flexible display device 100 including theadhesive film according to aspects of the present invention may includea photoelectric element part (91), a first adhesive film (92(a)) formedon the photoelectric element part (91), a touch function part (93)formed on the first adhesive film (92(a)), a second adhesive film(92(b)) formed on the touch function part (93) and a window film (95)formed on the second adhesive film (92(b)).

The photoelectric element part (91) may be any materials as long as theyare flexible materials, and be, for example, an OLED, a LED or a LCDincluding a light source. In some embodiments, a touch function part(93) may be formed on panels of the OLED, the LED or the LCD includingthe light source in the photoelectric element part (91), and the touchfunction part (93) may be bonded to a polarizer (94). The adhesive filmof the present invention may be interposed between panels of the OLED,the LED or the LCD including the light source and the touch functionpart (93).

The touch function part (93) may serve to sense electrical signalscaused by the contact with fingers, etc., and be formed of flexiblematerials. Specifically, metal nanowires of flexible conductivematerials, and conductors in which the conductive films including theconductive polymers are patterned may be used. For example, the touchfunction part (93) may include the conductor formed on the substratelayer (not shown). The touch function part (93) may be a typical one,available from the commercial sources. The photoelectric element part(91) may be laminated via the first adhesive film (92(a)) below thetouch function part (93).

The window film (95) may be a typical one, available from the commercialsources. In one embodiment, the window film (95) may be opticallytransparent, formed of the flexible resin, and consisting of a substratelayer and a hard coating layer. The substrate layer may be at least onepoly (meth)acrylate resin selected from, for example, a polyester resinsuch as polyethylene terephthalate, polyethylene naphthalate,polybutylene terephthalate, polybutylene naphthalate, and the like, apolycarbonate resin, a polyimide resin, a polystyrene resin, apolymethyl metacrylate, and the like. In addition, the hard coatinglayer may have a hardness of 6H or more in the pencil hardness test, andmay be formed of siloxane resin.

In one embodiment, the second adhesive film (92(b)) may be the samematerial as the first adhesive film (92(a)). Here, “same material” meansthat the components and physical properties are the same. In anotherembodiment, the second adhesive film (92(b)) may be different materialfrom the first adhesive film (92(a)). In still another embodiment, thethickness of the first adhesive film (92(a)) may be thicker than thesecond adhesive film (92(b)). Therefore, the first adhesive film (92(a))may have an adhesion higher than the second adhesive film (92(b)). Thus,it is possible to increase a reliability of the flexible display divice,and flatten the photoelectric element part.

According to the FIG. 2, in another embodiment, the flexible displaydevice (200 a, 200 b) may further include a polarizer (94) formed on thetouch function part (93) or the first adhesive film (92(a)). In oneembodiment, the polarizer (94) may be formed on the touch function part(93) in the flexible display device (200 a), and the window film (95)may be laminated via the second adhesive film (92(b)) of aspects of thepresent invention on the touch function part (93). Alternatively, thepolarizer (94) may be laminated via the first adhesive film (92(a)) onthe photoelectric element part (91) in the flexible display device (200b). In another embodiment, in the flexible display device (200 a), thesubstrate layer of the touch function part (93) may include thepolarizer, and in this case, the polarizer and the substrate layer ofthe touch function part (93) may be integrated.

The polarizer (94) may polarize a natural light or an artificial light,and may be prepared by dying an iodide or a dichromatic dye to amodified polyvinyl alcohol film such as a partial formylated polyvinylalcohol film, an acetoacetyl group modified polyvinyl alcohol film, andthe like or a polyolefin film and stretching it in a fixed direction.Specifically, it may be prepared through a swelling step, a dying step,and a stretching step. Each step will be typically known to thoseskilled in the art. The polarizer (94) may be any one, available fromthe commercial sources.

According to the FIG. 3, in still another embodiment, although the touchfunction part (93) of the flexible display device (300) is not shown,the conductors may be formed on the substrate layer, and the substratelayer may include the polarizer (98).

Next, the present invention will be described in more detail withreference to examples. However, it should be noted that these examplesare provided for illustration only and should not be construed in anyway as limiting the invention.

A description of details apparent to those skilled in the art will beomitted.

EXAMPLE Preparation of Adhesive Film

The components used for preparing an adhesive film are as follows:

(A) Monomer mixture:

(a1) 2-ethylhexyl acrylate (EHA) was used.

(a2) 4-hydroxybutyl acrylate (HBA) was used.

(B) Organic particles:

(b1) The organic particles, which had core-shell structures eachconsisting of a core of polybutyl acrylate (PBA) and a shell ofpolymethyl methacrylate (PMMA) present in an amount of 40 wt % in theorganic particles, and having an average particle diameter of 230 nm anda refractive index (N_(B)) of 1.48, were used.

(b2) The organic particles, which had core-shell structures eachconsisting of a core of polybutyl acrylate (PBA) and a shell ofpolymethyl methacrylate (PMMA) present in an amount of 30 wt % in theorganic particles, and having an average particle diameter of 230 nm anda refractive index (N_(B)) of 1.48, were used.

(b3) The organic particles, which had core-shell structures eachconsisting of a core of polybutyl acrylate (PBA) and a shell ofpolymethyl methacrylate (PMMA) present in an amount of 30 wt % in theorganic particles, and having an average particle diameter of 130 nm anda refractive index (N_(B)) of 1.48, were used.

(b4) The organic particles, which had core-shell structures eachconsisting of a core of poly 2-ethylhexyl acrylate (PEHA) and a shell ofpolymethyl methacrylate (PMMA) present in an amount of 30 wt % in theorganic particles, and having an average particle diameter of 140 nm anda refractive index (N_(B)) of 1.48, were used.

(C) Radical polymerizable initiator:

(c1) Irgacure 651 (2,2-dimethoxy-2-phenyl acetophenone) product fromBASF was used.

(c2) Irgacure 184 (1-hydroxycyclohexylphenyl ketone) product from BASFwas used.

(c3) AIBN (Azobisisobutyronitrile, Junsei Chemical Co., Ltd.) product asa thermal polymerizable initiator was used.

Preparation Example 1

100 parts by weight of a monomer mixture (A) containing 70 wt % of2-ethylhexyl acrylate (a1) and 30 wt % of 4-hydroxybutyl acrylate (a2),4 parts by weight of organic particles (b1) and 0.005 parts by weight ofa photo-polymerizable initiator (c1) (Irgacure 651) were mixed in aglass vessel to prepare a mixture. Oxygen dissolved in the glass vesselwas replaced with a nitrogen gas, and the mixture was polymerized byirradiating a UV light for few minutes using a low pressure lamp (BLLamp from Sankyo) to give a (meth)acrylic copolymer having a hydroxylgroup having a viscosity of 1000 CPS and a refractive index (N_(AB))shown in Table 1. 0.35 parts by weight of an additionalphoto-polymerizable initiator (c2) (Irgacure 184) was added to theresultant (meth)acrylic copolymer having the hydroxyl group to obtainthe pressure-sensitive adhesive composition.

The pressure-sensitive adhesive composition was coated to the polyesterfilm (release film, polyethylene terephthalate film, thickness of 50 μm)to give an adhesive film having a thickness of 100 μm. The upper side ofthe polyester film was covered with the release film having a thicknessof 75 μm, and both sides were irradiated for six minutes using a lowpressure lamp (BL Lamp from Sankyo) to give a transparent adhesivesheet.

Preparation Examples 2 to 7 and 9

The transparent adhesive sheets were prepared in the same manner as inPreparation Example 1 except that the contents of the component inPreparation Example 1 were changed as showed in Table 1.

Preparation Example 8

100 parts by weight of a monomer mixture (A) containing 80 wt % of2-ethylhexyl acrylate (a1) and 20 wt % of 4-hydroxybutyl acrylate (a2),4 parts by weight of organic particles (b3) and 0.05 parts by weight ofa photo-polymerizable initiator (AIBN (Junsei)), and 130 parts by weightof ethyl acetate based on 100 parts by weight were mixed in a glassvessel. Oxygen dissolved in a glass vessel was replaced with a nitrogengas, and the mixture was subjected to a typical polymerization to give a(meth)acrylic copolymer having a hydroxyl group having a refractiveindex (N_(AB)) shown in Table 1. 0.35 parts by weight of an additionalphoto-polymerizable initiator (c2) (Irgacure 184) was added to theresultant (meth)acrylic copolymer having the hydroxyl group to obtainthe pressure-sensitive adhesive composition.

Then, a transparent adhesive sheet was prepared using the same method asthe Preparation Example 1, and additionally air dried at 80° C. for 20minutes and at 100° C. for 5 minutes.

The physical properties for the transparent adhesive sheet prepared inPreparation Examples 1 to 9 were determined, and the results thereofwere shown in Table 1.

TABLE 1 Preparation Example 1 2 3 4 5 6 7 8 9 (A) (a1) 70 70 80 80 80 8080 80 70 (a2) 30 30 20 20 20 20 20 20 30 (B) (b1) 4 — — — 4 — — (b2) — 4— — — 4 — (b3) — — 4 — — — 2 4 — (b4) — — — 4 — — — (Meth)acrylic 1.471.47 1.48 1.48 1.48 1.48 1.48 1.48 1.47 copolymer having hydroxyl grouprefractive index (N_(AB)) |N_(AB) − N_(B)| 0.01 0.01 0 0 0 0 0 0 — (C)(c1) 0.005 0.005 0.005 0.005 0.005 0.005 0.005 — 0.005 (c2) 0.35 0.350.35 0.35 0.35 0.35 0.35 0.35 0.35 (c3) — — — — — — — 0.05 — Storage−20° C.   130 110 73 61 80 71 74 65 86 Modulus 25° C. 34 33 27 28 37 3527 25 22 (KPa) 80° C. 18 23 15 18 33 29 14 18 6 Average slope −1.12−0.87 −0.58 −0.43 −0.47 −0.42 −0.60 −0.47 −0.80 of −20° C. to 80° C.T-peel 25° C. 990 943 1073 881 1268 944 1105 984 483 strength 60° C. 536532 578 654 561 596 521 587 472 (gf/in) Haze (%) 0.78 0.49 0.51 0.440.92 0.49 0.28 0.52 0.45 Haze after 200% 0.97 0.62 0.55 0.45 1.26 0.670.37 0.54 0.47 stretching (%) Recovery rate (%) 79.5 78.0 67.2 49.1 82.574 50.0 77.5 38.4 Foaming area (%) 0 0 0 0 0 0 0 0 2.19 Elongation (%)970 1080 1580 1625 1105 1380 1250 1470 760

Method of Evaluating the Physical Properties

(1) Storage modulus: Viscoelasticity was measured at a shear rate of 1rad/sec at a strain of 1% under auto strain conditions using ARES(MCR-501, Anton Paar Co., Ltd.) which was a dynamic viscoelasticityinstrument. After removal of a release film, an adhesive film wasstacked to a thickness of 500 μm. Next, the stacked body was subjectedto punching using an 8 mm diameter puncher, thereby preparing aspecimen. Storage modulus was measured on the specimen at a temperatureof −60° C. to 90° C. at a heating rate of 5° C./min using an 8 mm jig,and storage modulus at each of −20° C., 25° C. and 80° C. was recorded.

(2) Average slope: The average slope at −20° C. to 80° C. for thedistribution of storage modulus versus temperature of the adhesive film,in which a temperature (° C.) is plotted as x-axis and a storage modulus(KPa) is plotted as y-axis, and is calculated according to Equation 3.

Average slope=(Mo(80° C.)−Mo(−20° C.))/(80−(−20))  [Equation 3]

(wherein Equation 3, Mo (80° C.) is a storage modulus (KPa) at 80° C.,and Mo (−20° C.) is a storage modulus (KPa) at −20° C.)

(3) T-peel strength: A PET film having a size of 150 mm×25 mm×75 μm(length×width×thickness) was subjected to corona treatment twice (totaldose: 156) under corona discharge at a dose of 78 using a coronatreatment device. An adhesive film sample having a size of 100 mm×25mm×100 μm (length×width×thickness) was obtained from each of theadhesive sheets of Examples and Comparative Example. Corona-treatedsurfaces of the PET films were laminated to both surfaces of theadhesive film sample, thereby preparing a specimen as shown in FIG.4(a). The specimen was subjected to autoclaving at a pressure of 3.5 barat 50° C. for 1,000 seconds and secured to a TA.XT_Plus texture analyzer(Stable Micro System Co., Ltd.). Referring to FIG. 4(b), the PET film atone side was kept fixed and the PET film at the other side was pulled ata rate of 50 mm/min at 25° C. using a TA.XT_Plus texture analyzer,thereby measuring T-peel strength at 25° C. (see FIG. 4(b)).

In addition, the PET film at one side was kept fixed and the PET film atthe other side was pulled at a rate of 50 mm/min at 60° C. using aTA.XT_Plus texture analyzer, thereby measuring T-peel strength at 60° C.

(4) Haze: Haze meter (Model NDH 5000 from Nippon Denshoku IndustriesCo., Ltd.) was used. The haze of thickness of 100 μm was determinedaccording to ASTM (American Society for Testing and Measurement) testmethod D 1003-95 5 (“Standard Test for Haze and Luminous Transmittanceof Transparent Plastic”).

(5) Haze after 200% stretching: Both ends of a sample (5 cm×5 cm,thickness: 100 μm) of the manufactured adhesive film were secured toboth sides of a horizontal tensile tester, followed by removing releasefilms from both surfaces of the sample. After the sample was subjectedto 200% stretching in a longitudinal direction (to a length twice aninitial length thereof, that is, a length of 10 cm), a glass plate wasplaced on a lower side of the sample and a release film was placed on anupper side of the sample, followed by bonding the sample to the glassplate through 2 kg rollers, thereby preparing a stretched specimen.Next, the release film was removed from the upper side, followed bymeasuring haze in the same manner as described above.

(6) Recovery rate: When both ends of each polyethylene terephthalate(PET) film (thickness: 75 μm) having a size of 50 mm×20 mm(length×width) were defined as a first end and a second end,respectively, a specimen was prepared by bonding ends of two PET filmsto each other via each of the adhesive films, which were prepared inExamples and a Comparative Example and had a size of 20 mm×20 mm(length×width), in order of first end of first PET film/adhesivefilm/second end of second PET film, and had a contact area of 20 mm×20mm (length×width) between each of the PET films and the adhesive film(see FIGS. 5(a) and 5 (b)). Referring to FIG. 5(a), jigs were secured tonon-bonded ends of the PET films of the specimen at room temperature(25° C.), respectively. Next, the jig at one side was kept fixed, andthe jig at the other side was pulled to a length of 1,000% of thickness(unit: μm) of the adhesive film of the adhesive film) at a rate of 300mm/min and then maintained for 10 seconds. Next, if an increased lengthof the adhesive film was defined as Xf (unit: μm) when a force of 0 kPawas applied to the adhesive film by recovering the adhesive film at thesame rate (300 mm/min) as the pulling rate, recovery rate (%) wascalculated by Equation 1.

Recovery rate (%)=(1−(Xf/X0))×100  [Equation 1]

(7) Elongation (%): The adhesive film of a size of 5 cm×5 cm was rolledclosely, and fixed to TA (TA.XT Plus Texture Analyzer (Stable MicroSystems, Ltd)), and then stretched at a rate of 300 mm/min to measurethe ratio of the length change.

(8) Foaming area of adhesive film (%): The adhesive film, with PEThaving a thickness of 50 μm laminated on the one side of the adhesivelayer (13 cm×3 cm, thickness 100 μm) and PET having a thickness of 100μm laminated on the other side of the adhesive layer, was bended andputted into the parallel frames having a spacing of 1 cm in thedirection of PET having a thickness of 50 μm such that the width lengthof the specimen of the adhesive film became ½, and aged at temperatureof 70° C. and humidity of 93% for 24 hours. Then, the images of thearea, in which the bubbles occur, taken with an optical microscope(Olympus, EX-51, 30×) are analyzed using a Mac-view software fromMountech Co., Ltd., to calculate a ratio (%) of the area and size inwhich the bubbles occupy to the area of the specimen.

Example 1

The flexible display device (13 cm×3 cm) was prepared by laminating thefollowing layers.

-   -   Photoelectric element part: It was replaced with PET (thickness        of 100 μm).    -   First adhesive film: The adhesive film having a thickness of 100        μm was formed from the composition of the Preparation Example 1.    -   PC film (touch function part) with a transparent electrode        applied: The film was prepared by applying the silver nanowire        containing solution (Clearohm-A G4-05, Cambrios Technologies        Corporation, containing silver nanowire and binder) to Teijin        chemical PC film having a thickness of 50 μm.    -   Second adhesive film: The adhesive film having a thickness of 50        μm was formed from the composition of the Preparation Example 1.    -   Window film: It was replaced with PET (100 μm Cosmoshine TA015,        Toyobo Co., Ltd., thickness: 100 μm).

Examples 2 to 8

The flexible display devices were prepared in the same manner as inExample 1 except that the pressure-sensitive adhesive compositions forpreparing the adhesive film in Preparation Examples 2 to 8 were used.

Comparative Example 1

The flexible display device was prepared in the same manner as inExample 1 except that the pressure-sensitive adhesive composition forpreparing the adhesive film in Preparation Example 9 was used.

The physical properties for Examples 1 to 8 and Comparative Example 1were determined, and the results thereof were shown in Table 2.

TABLE 2 Comparative Examples Example 1 2 3 4 5 6 7 8 1 Foaming area (%)0 0 0 0 0 0 0 0 3.22

Method of Evaluating the Physical Properties

Foaming area of display device (%): The prepared display devices werebended and putted into the parallel frames having a spacing of 1 cm inthe direction of PET (substituted for the photoelectric element part)having a thickness of 100 μm, and aged at temperature of 70° C. andhumidity of 93% for 24 hours. Then, the images of the display devicetaken with an optical microscope (Olympus, EX-51, 30×) was analyzedusing a Mac-view software from Mountech Co., Ltd., to calculate a ratio(%) of the area and size in which the bubbles occupy to the total area.

Example embodiments have been disclosed herein, and although specificterms are employed, they are used and are to be interpreted in a genericand descriptive sense only and not for purpose of limitation. In someinstances, as would be apparent to one of ordinary skill in the art asof the filing of the present application, features, characteristics,and/or elements described in connection with a particular embodiment maybe used singly or in combination with features, characteristics, and/orelements described in connection with other embodiments unless otherwisespecifically indicated. Accordingly, it will be understood by those ofskill in the art that various changes in form and details may be madewithout departing from the spirit and scope of the present invention asset forth in the following claims.

1. A flexible display device comprising: a photoelectric element part; afirst adhesive film formed on the photoelectric element part; a touchfunction part formed on the first adhesive film; a second adhesive filmformed on the touch function part; and a window film formed on thesecond adhesive film; wherein the first adhesive film or the secondadhesive film has an average slope of about −9.9 to about 0 at −20° C.to 80° C. based on x-axis for the distribution of storage modulus versustemperature in the function of x-axis of temperature (° C.) and y-axisof storage modulus (KPa), and wherein the first adhesive film or thesecond adhesive film has a storage modulus of 10 KPa or more at 80° C.2. A flexible display device comprising: a photoelectric element part; afirst adhesive film formed on the photoelectric element part; a touchfunction part formed on the first adhesive film; a second adhesive filmformed on the touch function part; and a window film formed on thesecond adhesive film; wherein the first adhesive film or the secondadhesive film is formed from an pressure-sensitive adhesive compositioncomprising a monomer mixture for a (meth)acrylic copolymer having ahydroxyl group; and organic particles, and the organic particles have anaverage particle diameter of about 10 nm to about 400 nm.
 3. Theflexible display device according to claim 1, wherein the flexibledisplay device further comprises a polarizer formed on the touchfunction part or on the first adhesive film.
 4. The flexible displaydivice device according to claim 1, wherein the touch function partcomprises a substrate layer, and the substrate layer comprises apolarizer.
 5. The flexible display device according to claim 1, whereinthe photoelectric element part is an OLED, a LED or a LCD comprising alight source.
 6. The flexible display device according to claim 1,wherein the first adhesive film has a thickness greater than that of thesecond adhesive film.
 7. The flexible display device according to claim1, wherein the first adhesive film or the second adhesive film may havea T-peel strength of about 400 gf/in to about 5,000 gf/in, as measuredat 25° C. with respect to a corona-treated polyethylene terephthalate(PET) film.
 8. The flexible display device according to claim 1, whereinthe first adhesive film or the second adhesive film may have a T-peelstrength of about 200 gf/in to about 3,000 gf/in, as measured at 60° C.with respect to a corona-treated polyethylene terephthalate (PET) film.9. The flexible display device according to claim 2, wherein the firstadhesive film or the second adhesive film has a storage modulus of about10 KPa to about 1000 KPa at 80° C.
 10. The flexible display deviceaccording to claim 1, wherein the first adhesive film or the secondadhesive film has a storage modulus of about 10 KPa to about 1000 KPa at−20° C.
 11. The flexible display divice device according to claim 1,wherein the first adhesive film or the second adhesive film has a hazeof about 5% or less after 200% stretching at a thickness of 100 μm. 12.The flexible display device according to claim 1, wherein the firstadhesive film or the second adhesive film has a recovery rate of about30% to about 98% at a thickness of 100 μm according to the followingEquation 1:Recovery rate (%)=(1−(Xf/X0))×100,  [Equation 1] where in Equation 1, X0and Xf are defined as follows: when both ends of each of polyethyleneterephthalate (PET) films (thickness: about 75 μm) having a size ofabout 50 mm×about 20 mm (length×width) are defined as a first end and asecond end, respectively, a specimen is prepared by bonding ends of twoPET films to each other via an adhesive film having a size of about 20mm×about 20 mm (length×width) in order of first end of first PETfilm/adhesive film (length×width: about 20 mm×about 20 mm)/second end ofsecond PET film; next, jigs are secured to non-bonded ends of the PETfilms of the specimen, respectively; next, the jig at one side is keptfixed and the jig at the other side is pulled to a length of 1,000% ofthickness (unit: μm) of the adhesive film of the adhesive film) at arate of about 300 mm/min and then maintained for about 10 seconds;wherein when a force of 0 kPa is applied to the adhesive film byrecovering the adhesive film at the same rate (about 300 mm/min) as thepulling rate, an increased length of the adhesive film is defined as Xf(unit: μm)).
 13. The flexible display device according to claim 1,wherein the first adhesive film or the second adhesive film has anelongation of about 800% to about 2000%.
 14. The flexible display deviceaccording to claim 1, where in the first adhesive film or the secondadhesive film may have a bubble generation area of about 0%, as measuredwhen the adhesive film (length×width×thickness: about 13 cm×about 3cm×about 100 μm) comprising a about 50 μm thick PET film stacked on onesurface thereof and a about 100 μm thick PET film stacked on the othersurface thereof is bent towards the about 50 μm thick PET film such thatthe adhesive film has about ½ of the length, followed by placing theadhesive film between parallel frames having a gap of about 1 cm, andthen subjected to aging under conditions of about 70° C. and humidity ofabout 93% for about 24 hours.
 15. The flexible display device accordingto claim 1, where in the first adhesive film or the second adhesive filmin the flexible display device may have a bubble generation area ofabout 0%, as measured when the flexible display device that a windowfilm was replaced with PET film (thickness of about 100 μm), bended intothe parallel frames having a spacing of about 1 cm in the direction ofthe photoelectric element part, and aging at temperature of about 70° C.and humidity of about 93% for about 24 hours.
 16. The flexible displaydevice according to claim 1, wherein the first adhesive film or thesecond adhesive film is a cured product of the pressure-sensitiveadhesive composition, and the pressure-sensitive adhesive compositioncomprises a monomer mixture for a (meth)acrylic copolymer having ahydroxyl group, and organic particles.
 17. The flexible display deviceaccording to claim 16, wherein the organic particles have an averageparticle diameter of about 10 nm to about 400 nm.
 18. The flexibledisplay device according to claim 16, wherein the monomer mixturecomprise a (meth)acrylate monomer having the hydroxyl group and acomonomer.
 19. The flexible display device according to claim 16,wherein the organic particles have core-shell structures, and a glasstransition temperature of the core and the shell satisfies the followingEquation 2:Tg(c)<Tg(s)  [Equation 2] wherein, Tg(c) is a glass transitiontemperature (° C.) of the core, and Tg(s) is a glass transitiontemperature (° C.) of the shell.
 20. The flexible display deviceaccording to claim 19, wherein the core has a glass transitiontemperature of about −150° C. to about 10° C., and the shell has a glasstransition temperature of about 15° C. to about 150° C.
 21. The flexibledisplay device according to claim 19, wherein the core comprises atleast one polyalkyl (meth)acrylate having a glass transition temperatureof about −150° C. to about 10° C., and the shell comprises at least onepolyalkyl (meth)acrylate having a glass transition temperature of about15° C. to about 150° C.
 22. The flexible display device according toclaim 19, wherein the shell comprises two or more of layers, and theoutermost layer of the organic particles comprises at least onepolyalkyl (meth)acrylate having a glass transition temperature of about15° C. to about 150° C.
 23. The flexible display device according toclaim 19, wherein the shell is present in an amount of about 1 wt % toabout 70 wt % in the organic particles.
 24. The flexible display deviceaccording to claim 16, wherein the organic particles are present in anamount of about 0.1 parts by weight to about 15 parts by weight based on100 parts by weight of the monomer mixture containing the (meth)acryliccopolymer having the hydroxyl group.
 25. The flexible display deviceaccording to claim 16, wherein the organic particles have a refractiveindex difference of about 0.05 or less from that of the (meth)acryliccopolymer having the hydroxyl group.
 26. The flexible display deviceaccording to claim 25, wherein the monomer mixture containing the(meth)acrylic copolymer having the hydroxyl group comprises about 60 wt% to about 95 wt % of the comonomer and about 5 wt % to about 40 wt % ofthe (meth)acrylate monomer having the hydroxyl group.
 27. The flexibledisplay device according to claim 18, wherein the comonomer comprises atleast one of an alkyl (meth)acrylate monomer, a monomer having ethyleneoxide, a monomer having propylene oxide, a monomer having amine group, amonomer having amide group, a monomer having alkoxy group, a monomerhaving phosphate group, a monomer having sulfonic acid group, a monomerhaving phenyl group and a monomer having silane group, and the comonomerhas a glass transition temperature (Tg) of about −150° C. to about 0° C.28. The flexible display device according to claim 16, wherein thepressure-sensitive adhesive composition further comprises at least oneof an initiator and a crosslinking agent.